It has been proposed to provide the thermoplastifiable material which is to be subjected to extrusion in a worm-type extrusion press, to preheating before this material is admitted to the extruder and thereby contribute to the material some of the heat required for the plastification.
Such techniques permit the output of an extrusion press to be increased or the extrusion press to be better utilized.
The charging or metering device may also be utilized to ensure an exact rate of feed of the material to the extrusion press and thereby maintain an output which is at or close to maximum without danger of overloading the extrusion press and when a portion of the heat is contributed by the charging device, permitting control of the temperature within the press by ensuring that the material will enter with a certain preheat temperature.
The preheating and charging device must be designed, therefore, to be capable of preheating the material, feed the material with precision and, above all, to ensure a uniform temperature of the preheated material fed to the extrusion press.
In other words, temperature differences within the synthetic resin material to be processed should be avoided as much as possible and the material should be able to be brought to a temperature close to that at which the material becomes sticky and plastifiable.
When these conditions can be maintained, it is also necessary to ensure that the material can be fed to a given extrusion press without significant modification thereof and with precise control of the feed rate and the preheating temperature, so that the production of thermoplastic products with the sharply varying characteristics due to preheating temperature fluctuations, temperature differences in the preheated material and variations in the feed rate can be avoided.
It has been proposed to provide a charging device for an extrusion press which comprises at least two worms journaled in a heated housing.
The first worm carries the material out of a feed funnel while the second worm delivers the material to the extruder feed passage. The device is designed so that particles of material are carried through both worms a plurality of times. This known arrangement with lateral preheating and material circulation allows preheating of the material practically independently of the volume rate of feed thereof but has the drawback that the material is not uniformly heated. In addition, this worm preheater has the disadvantage that a metering device is required between the preheater and the extrusion press.